A conventional spark plug is disclosed in Japanese Patent Application Laid-Open (kokai) No. H11-185928 (Patent Document 1). The spark plug is comprised of a ground electrode having a base end portion fixed to a metal shell, a bend portion integrally formed with the base end portion and being bent, and a front end portion integrally formed with the bend portion and forming a spark discharge gap with a center electrode.
The ground electrode is comprised of: a core extending from the base end portion towards the front end portion through the bend portion; a heat transfer portion disposed outside of the core and extending from the base end portion towards the front end portion through the bend portion; and an outer layer disposed outside of the heat transfer portion and extending from the base end portion up to the front end portion through the bend portion.
The core is made of pure nickel, and the heat transfer portion is made of copper, and the outer layer is made of a nickel base alloy. The pure nickel constituting the core has Vickers hardness Hv of 96, which is higher than that of the copper of 46. The copper constituting the heat transfer portion has thermal conductivity of 0.94 cal/cm·sec·° C., which is greater than that of the nickel base alloy. Further, the copper constituting the heat transfer portion has a coefficient of thermal expansion of 17.0×10−6/° C., which is greater than that of the nickel base alloy of 11.5×10−6/° C. and that of the pure nickel of 13.3×10−6/° C. The nickel base alloy constituting the outer layer has better heat resistance and corrosion resistance than those of copper and pure nickel.
The conventional spark plug having such composition is mounted on an engine and repeatedly discharges between the center electrode and the ground electrode under high temperature conditions.
In this spark plug, since the copper constituting the heat transfer portion has excellent thermal conductivity, the heat in the front end portion is effectively conducted to the base end portion through the heat transfer portion, whereby the heat can be properly transferred from the metal shell to the engine. That is, this spark plug can prevent the heat rise in the front end portion and exhibit outstanding durability as the heat transfer portion has excellent thermal conduction.
However, since the spark plug has the heat transfer portion in which the copper constituting the heat transfer portion has large coefficient of thermal expansion, the ground electrode tends to be lifted up under high temperature conditions. When the ground electrode is lifted up, the spark discharge gap formed between the ground electrode and the center electrode varies. This variation causes adverse effect on the ground electrode. Thus, the spark plug is capable of preventing this lift-up phenomenon of the ground electrode by adjusting the thicknesses of the heat transfer portion and the outer layer. Further, since the hardness of the pure nickel constituting the core is higher than that of the copper constituting the heat transfer portion contributes to the prevention of the lift-up phenomenon of the ground electrode.